For those of you who were wondering why the upstairs sink was out of commission, a visual:
This hole was in the pipe behind the wall. No wonder it was leaking!
The plumber went in from the back side of the wall, as so not to damage my pretty tile (which I very much appreciate). And he replaced the whole trap assembly, so what is seen on the front side is now nice & shiny. Now the upstairs bathroom is fully functional. Hooray, plumber dude!